The present invention relates generally to a passive cooling device for dissipating heat from a component connected with the passive cooling device. More specifically, the present invention relates to a passive cooling device for dissipating heat from a component connected with the passive cooling device and including a duct for channeling an air flow between an air flow source and the passive cooling device.
It is well known in the electronics art to place a heat sink in contact with an electronic device so that waste heat generated by operation of the electronic device is thermally transferred into the heat sink thereby cooling the electronic device. With the advent of high clock speed electronic devices such as microprocessors (xcexcP), digital signal processors (DSP), and application specific integrated circuits (ASIC), the amount of waste heat generated by those electronic devices and the operating temperature of those electronic devices are directly proportional to clock speed. Therefore, higher clock speeds result in increased waste heat generation which in turn increases the operating temperature of the electronic device. However, efficient operation of the electronic device requires that waste heat be continuously and effectively removed.
Heat sink devices came into common use as a preferred means for dissipating waste heat from electronic devices such as the types described above. In a typical application, a component to be cooled is carried by a connector that is mounted on a PC board. A heat sink is mounted on the component by attaching the heat sink to the connector using a clip or fasteners, for example. Alternatively, the heat sink is mounted to a PC board that carries the electronic device and fasteners or the like are used to connect the heat sink to the PC board via holes that are drilled in the PC board.
Typically, a heat sink used in conjunction with a modern high clock speed electronic device will include an electrical fan mounted on top of the heat sink or within a cavity formed by cooling fins/vanes of the heat sink. The cooling fins increase the surface area of the heat sink and maximize heat transfer from the heat sink to ambient air that surrounds the heat sink. The fan causes air to circulate over and around the cooling fins thereby transferring heat from the cooling fins into the ambient air.
As mentioned previously, with continuing increases in clock speed, the amount of waste heat generated by electronic devices has also increased. Accordingly, to adequately cool those electronic devices, larger heat sinks and/or larger fans are required. Increasing the size of the heat sink results in a greater thermal mass and a greater surface area from which the heat can be dissipated. A larger fan increases air flow through the cooling fins.
There are disadvantages to increased fan and heat sink size. First, if the size of the heat sink is increased in a vertical direction (i.e. in a direction transverse to the PC board), then the heat sink is tall and may not fit within a vertical space in many applications, such as the chassis of a desktop computer. Second, if the PC board has a vertical orientation, then a tall and heavy heat sink can mechanically stress the PC board and/or the electronic device resulting in a device or PC board failure.
Third, a tall heat sink will require additional vertical clearance between the heat sink and a chassis the heat sink is contained in to allow for adequate air flow into or out of the fan. Fourth, if the heat sinks size is increased in a horizontal direction, then the amount of area available on the PC board for mounting other electronic devices is reduced. Fifth, when the heat sink has a cylindrical shape formed by the fins it is often not possible to mount several such heat sinks in close proximity to each other because air flow into and out of the fins is blocked by adjacent heat sinks with a resulting decrease in cooling efficiency.
Finally, increases in fan size to increase cooling capacity often result in increased noise generation by the fan. In many applications such as the desktop computer or a portable computer, it is highly desirable to minimize noise generation. In portable applications that depend on a battery to supply power, the increased power drain of a larger fan is not an acceptable solution for removing waste heat.
In the above mentioned heat sink with cooling fins there are additional disadvantages to mounting the fan within a cavity formed by the fins. First, a substantial portion of a heat mass of the heat sink is partially blocked by the fan because the fan is mounted directly on the heat mass and therefore blocks a potential path for heat dissipation from the heat mass because air from the fan does not circulate over the blocked portion of the heat mass.
Second, without the fan, a depth of the fins could extend all the way to a center of the heat mass; however, the depth and surface area of the fins is reduced by a diameter of the fan because the fan is mounted in a cavity having a diameter that is slightly larger than the fans diameter to provide clearance for the fan blades. Consequently, the heat mass of the heat sink must be made broader to compensate for the reduced surface area of the fins. The broader heat mass increases the size, cost, and weight of the heat sink.
Third, the reduced depth of the fins makes it easier for the fins to be bent if damaged. One possible consequence of a bent fin is that it will contact and damage the fan blades and/or cause the fan to stall thereby damaging the fan or causing the fan to fail. Fourth, because the fan is mounted in the cavity formed by the fins, power leads for the fan must be routed through a space between the fins. Sharp edges on the fins can cut the power leads or cause an electrical short. In either case, the result is that the fan will fail. Fifth, glue is typically used to mount the fan to the heat sink and the glue can get into the fan and cause the fan to fail. Any of the above mentioned failure modes can lead to a failure of the electronic device the heat sink was designed to cool because air circulation generated by the fan is essential to effectively dissipate waste heat from the electronic device.
Lastly, the fan itself is typically connected with the fins of the heat sink or is positioned in a cavity between the fins. Therefore, the heat sink is actively cooled by a direct connection with the fan that generates an air flow through the fins. Because of the proximity of the fan with the fins, the air flow is turbulent and that turbulence generates noise in addition to the noise generated by the operation of the fan itself.
Consequently, there exists a need for a passive cooling device that overcomes the aforementioned disadvantages associated with prior actively cooled fan assisted heat sinks. Moreover, there is a need for a passive cooling device that receives an air flow from an air flow source that is not directly connected with the passive cooling device. Finally, there is a need for a passive cooling device adapted to receive a laminar air flow that efficiently dissipates waste heat at reduced noise levels.
Broadly, the present invention is embodied in a passive cooling device for dissipating heat from a component to be cooled. The passive cooling device includes a heat mass, a stem connected with the heat mass and extending outward of the heat mass. The stem is symmetrically positioned about an axis of the heat mass and the stem includes a diverter surface, an end face, and a plurality of stem fins that are spaced apart to define a stem slot between adjacent stem fins. The stem slots extend to the heat mass. A heat conductive base is in contact with the heat mass and includes a mounting surface that is adapted to thermally connect the heat mass with the component.
A plurality of vanes are connected with the heat mass and are spaced apart to define a primary slot between adjacent vanes. The primary slot extends to the heat mass. Each vane also includes at least one secondary slot therein that extends through a portion of the vane and defines a plurality of fins in each vane.
Each vane also includes an inner wall that terminates at a top face. The inner wall of all the vanes defines a chamber that surrounds the stem. A portion of the inner wall and the stem defines a cavitation section. Each vane further includes an outer wall that also terminates at the top face.
The passive cooling device further includes an air flow source for generating an air flow and a duct for channeling the air flow between the air flow source and the passive cooling device. The duct includes a first opening positioned adjacent to the top face and a second opening adapted to connect with the air flow source. Heat is dissipated from the component by the air flow passing over the stem, the stem fins, the fins, and the vanes and flowing through the stem slots, the primary slots, and the secondary slots such that heat conducted from the component and into the heat mass is transferred into the air flow.
Because the air flow source is not directly connected with the passive cooling device, the fan blades will not be damaged by a bent fin. A wire, such as a power lead for the fan, will not be cut or shorted by the vanes because the power lead need not be routed through the vanes and is positioned away from the vanes.
Moreover, the vanes of the present invention can extend deep into the heat mass because the fan is not in contact with the heat mass and is not positioned between the vanes so that the heat mass is not blocked by the fan. As a result, a surface area of the vanes can be increased over prior heat sink designs and that increased surface area improves heat dissipation from the heat mass.
The aforementioned problems associated with fan noise and air turbulence are also reduced by the present invention because the air flow in the duct reduces noise caused by air turbulence and because the fan blades are positioned away from the vanes and fins thereby further reducing air turbulence noise.